1. Field of the Invention
This invention relates to a control for maintaining the position of an impact spot of an electron beam on a workpiece, and, more particularly, to such a control that utilizes X-rays emanating from the impact spot to adjust the direction of the electron beam so that the actual impact spot is coincident with its reference position.
2. Description of the Prior Art
Electron beam welding is generally performed in the interior of a vacuum chamber so that during the welding process the position of the focused electron beam on the workpiece being welded must be had by observation through windows in the vacuum chamber walls. Welding that employs the use of an electron beam requires that the beam impact the workpiece at a point that is on the weld seam between the parts being welded.
Electron beam welding has heretofore been limited by the ability of a control system to maintain an accurate control of the position of the impact spot in relation to a predetermined path that the weld is intended to take. By means of electric or magnetic field the beam can be deflected to follow any pre-programmed path, the deflection fields being pre-programmed by, for example, a computer, just as in any cathode ray oscilloscope or in a television tube. However, the programmed path may not coincide with the intended weld path for several reasons: there may be an error in the command signal; the beam's response may change; spurious electrical and magnetic fields may cause unintended additional deflections; and the joint line between the work pieces may differ from its presumed path in order to overcome the difficulties in controlling the position of the impact spot and to maintain its position precisely on the joint line between the parts to be welded, various control techniques have been attempted and are known from the prior art. Typical of these is an optical control system wherein optical means are placed in the path of light emanating from the welding spot in order to observe the position of the welding spot in its relationship to the parting line. The observed light radiation is detected by an optical electrical conversion device to obtain electrical signals representative of the position of the welding spot. These electrical signals are then used to control and to adjust the position of the impact spot on the workpiece. Control systems employing optical means of this sort are set out in U.S. Pat. Nos. 3,766,355 and 3,471,703. Optical control systems are encumbered by the need for the detector to have an unobscured view of the impact spot. Unfortunately, this spot is obscured in actual practice by the surrounding incandescent glow at the impact spot and by the metal vapor produced during the welding process in the immediate vicinity of the impact spot.
Magnetic and electrostatic beam sensors have been employed to position the impact spot on the workpiece, but systems employing this approach require the use of pulsed beams. They are, therefore, not applicable to control DC welding beams. Disclosure of control systems of the magnetic sensor type are described in U.S. Pat. Nos. 3,496,463 and 3,152,238.
A third means known in the prior art for controlling the position of the impact spot is the use of electron beam scanning control systems. Scanning systems have been described in the following U.S. Pat. Nos. 3,513,285 and 3,573,358, RE 27,005 and 3,609,288. Scanning systems require that the beam power be reduced below the levels required for welding. As a result of this the impact spot produced with the lower power beam may not be located at the same point as the impact spot produced by the higher power beam. Prescanning before the actual welding process requires the storage of data, the necessary computer memory and later recall of the data during the actual welding process so that the reference position can be related to the actual position of the weld spot. In the time interval between scanning the workpiece and the welding operation, all inactive parameters must remain extremely stable. The electronic stabilization of high voltage, at high power and variable power, as is typically used in electron beam welding, requires complex and expensive electronic circuits. With our system inherent beam stability is much less critical.